A device may have input means that can be used to receive transmitted signals from the surrounding environment. For example, a device may have audio input means such as a microphone that can be used to receive audio signals from the surrounding environment. For example, a microphone of a user device may receive a primary audio signal (such as speech from a user) as well as other audio signals. The other audio signals may be interfering audio signals received at the microphone of the device, and may be received from an interfering source or may be ambient background noise or microphone self-noise. The interfering audio signals may disturb the primary audio signals received at the device. The device may use the received audio signals for many different purposes. For example, where the received audio signals are speech signals received from a user, the speech signals may be processed by the device for use in a communication event, e.g. by transmitting the speech signals over a network to another device which may be associated with another user of the communication event. Alternatively, or additionally, the received audio signals could be used for other purposes, as is known in the art.
In other examples, a device may have receiving means for receiving other types of transmitted signals, such as general broadband signals, general narrowband signals, radar signals, sonar signals, antenna signals, radio waves or microwaves. The same situations can occur for these other types of transmitted signals whereby a primary signal is received as well as interfering signals at the receiving means. The description below is provided mainly in relation to the receipt of audio signals at a device, but the same principles will apply for the receipt of other types of transmitted signals at a device, such as general broadband signals, general narrowband signals, radar signals, sonar signals, antenna signals, radio waves and microwaves as described above.
In order to improve the quality of the received audio signals, (e.g. the speech signals received from a user for use in a call), it is desirable to suppress interfering audio signals (e.g. background noise and interfering audio signals received from interfering audio sources) that are received at the microphone of the user device.
The use of stereo microphones and other microphone arrays in which a plurality of microphones operate as a single audio input means is becoming more common. The use of a plurality of microphones at a device enables the use of extracted spatial information from the received audio signals in addition to information that can be extracted from an audio signal received by a single microphone. When using such devices one approach for suppressing interfering audio signals is to apply a beamformer to the audio signals received by the plurality of microphones. Beamforming is a process of focussing the audio signals received by a microphone array by applying signal processing to enhance particular audio signals received at the microphone array from one or more desired directions. For simplicity we will describe the case with only a single desired direction herein, but the same method will apply when there are more directions of interest. The angle from which the desired audio signal is received at the microphone array, so-called Direction of Arrival (“DOA”) information can be determined or set prior to the beamforming process. It can be advantageous to set the desired direction of arrival to be fixed since the estimation of the direction of arrival may be complex. However, in alternative situations it can be advantageous to adapt the desired direction of arrival to changing conditions, and so it may be advantageous to perform the estimation of the desired direction of arrival in real-time as the beamformer is used. Adaptive beamformers apply beamformer coefficients to the received audio signals whereby they use the DOA information to process the audio signals received by the plurality of microphones to form a “beam” whereby a high gain is applied in a direction from which the desired audio signal is received by the microphones and a low gain is applied in the direction of any interfering sources.
While the beamformer will attempt to suppress the unwanted audio signals coming from unwanted directions, the number of microphones as well as the shape and the size of the microphone array will limit the effect of the beamformer, and as a result the unwanted audio signals are suppressed, but may remain audible. The output of the beamformer can be further processed in the device in the same way as a received audio signal from a single microphone may be processed, e.g. for transmission to another device as part of a communication event. For example, the output of the beamformer may be supplied as an input signal to at least one of an echo cancellation stage, an Automatic Gain Control (AGC) processing stage and a single channel noise reduction stage in the device.
It may be useful to determine particular directions of arrival at the microphones relating to particular interfering audio signals coming from particular interference sources. It may then be beneficial to adapt the beamformer coefficients of the beamformer to apply a greater level of suppression to audio signals which are received from a particular direction of known interference (i.e. an interfering direction of arrival). In this way, the interference of the primary (or “desired”) audio signal can be reduced.
The directions of arrival for sources of interference may be determined by calculating the cross-covariance between the input audio signals to determine the direction of arrival for the audio signals. The cross-covariance indicates the direction of arrival for which the audio signals received at the plurality of microphones are most highly correlated.